Iodine is a substance which readily becomes an anion, and is one of suitable positive electrode active substances. Iodine is known to form charge transfer complexes with various organic compounds. The charge transfer complexes of iodine are composed of iodine (acceptor) and various electron donating organic compounds (organic donor component). Known electron-donating compounds include heterocyclic compounds such as phenothiazine and carbazole; polynuclear aromatic compounds such as pyrene and perylene; and organic polymers such as poly-2-vinylpyridine, polyacetylene, poly-p-phenylene, polythienylene, polypyrrole, polyethers, polyaniline, polyamides, polyvinyl alcohol, polyethylene, polypropylene and polystyrene.
Some cells are known in which such an iodine charge transfer complex (to be referred to as the iodine complex or simply the complex adduct) is used as a main component of a positive electrode material. Furthermore, the present inventors found that such organic polymers as urea-formaldehyde resin, polyurethane, polyacrylonitrile, polyacrylamide, polymethacrylamide, polyethers and melamine resins are effective as the organic donor component.
However, the cells which use such iodine complexes as positive electrode active substances have problems. They have a low output. As discharging proceeds, they lose iodine, and abruptly decrease in electrical conductivity. Consequently, the resistance of the positive electrode material increases very much, and the internal resistances of the cells increase remarkably.
When a positive electrode material composed of the iodine complex is used in a secondary cell, discharging causes most of iodine to come off from the positive electrode material. Hence, this constitutes a decisive defect in secondary cells, i.e. the defect that the positive electrode material usually becomes an insulator and makes it impossible to recharge the cells. This problem inevitably arises when using iodine complexes of polymers which are intrinsically insulators, such as polyamides, polyacrylonitrile, polyvinyl alcohol, polyethers and poly-2-vinylpyridine.
Furthermore, such cells have the defect of short service lives.